The following is a slight rewording of an example in Richard Thaler's Misbehaving (1):
Positive framing of certain outcome
Imagine you are $300 richer than you are today. You are given a choice between:
A. A certain gain of \$100.
B. A 50% chance to gain \$200 and a 50% chance of losing \$0.
Negative framing of certain outcome
Imagine you are $500 richer than you are today. You are given a choice between:
A. A certain loss of \$100.
B. A 50% chance of losing \$200 and a 50% chance of losing \$0.
It can be seen that, under either framing, choice A implies \$400 richer than today while choice B implies a 50% chance of \$500 richer and a 50% chance of \$300 richer. According to Thaler, however, 72% of subjects chose A when presented with the positive framing and only 36% chose it when presented with the negative framing.
To modify this to fit the terms of your question, the initial figures in both framings could be reduced by \$300. The second would then become "Imagine you are \$200 richer ...". The first would no longer require any imagining. My (untested) intuition, however, is that the inclusion of imagining in one framing but not in the other might influence people's responses in some way that Thaler's formulation avoids.
I therefore suggest instead the following, because it retains the symmetry of imagining in both framings:
Positive framing of certain outcome
Imagine you are \$50 richer than you are today. You are given a choice between:
A. A certain gain of \$50.
B. A 50% chance to gain \$150 and a 50% chance to lose \$50.
Negative framing of certain outcome
Imagine you are \$150 richer than you are today. You are given a choice between:
A. A certain loss of \$50.
B. A 50% chance to gain \$50 and a 50% chance to lose \$150.
Reference
(1) Thaler R (2015) Misbehaving: the Making of Behavioural Economics, republished by Penguin Random House UK in 2016, p 33.
